1. Field of the Invention
This invention relates to an improved process for the esterification of organic acids, and, more specifically, this invention relates to an improved process for the direct esterification of ammonium salts of organic acids.
2. Background of the Invention
Environmental concerns call for the development of relatively non-toxic, biodegradable solvents for paint and grease removal. Ethyl lactate is a non-toxic biodegradable solvent that solubilizes several paint resins.
Esterification of organic acids with alcohol is a reversible process, as depicted in Equation 1, infra.
RCOOH+Rxe2x80x2OH⇄RCOORxe2x80x2+H2Oxe2x80x83xe2x80x83Eq. 1
where RCOOH is a carboxylic acid and Rxe2x80x2OH is an alcohol. Shifting of the equilibrium to the right is achieved by the removal of water produced in the reaction. Simple distillation of the water is effective where none of the other components (starting materials and ester) boils at a temperature below or close to the boiling point of water.
In many cases, carboxylic acid salts and lower boiling reactants must be utilized to produce specific size lactates. The water distillation process described supra does not accommodate some of these lower-boiling reactants. As an alternative, low temperature processes to produce ester compounds such as ethyl lactate exist. However, these low temperature processes tend to give low yields and are expensive. Given this current state of the art, a greater utilization of esters as organic solvents will not occur without a substantial reduction in the cost of production.
One route to the production of ethyl lactate is via the utilization of ammonium lactate as a feedstock. Ammonium lactate is a very desirable and inexpensive reactant because lactic acid and ammonium lactate are produced on a commercial scale by fermentation of carbohydrates during, for example, the production of cheese from milk. The least expensive process for producing ethyl lactate is directly from ammonium or calcium lactate, via the reactions depicted in Equations 2 and 3, respectively:
RCOOxe2x88x92NH4++Rxe2x80x2OHxe2x86x92RCOORxe2x80x2+H2O+NH3xe2x80x83xe2x80x83Eq. 2
(RCOOxe2x88x92)2Ca2++2Rxe2x80x2OHxe2x86x922RCOORxe2x80x2+Ca(OH)2xe2x80x83xe2x80x83Eq. 3
wherein R is CH3(OH)CH, and Rxe2x80x2 is CH3CH2. As can be noted in Table 1, infra, the desired esterification rate is extremely low.
Another drawback to using ammonium lactate as a feedstock is that relatively large amounts of it are converted to the undesired lactamide. Table 1 displays conversion and selectivity results for such a situation. The formation of lactamide from the attempted conversion of ammonium lactate to ethyl lactate lowers the selectivity of the reaction, causes loss of desired product, requires extra separation steps, and forms additional chemical waste which has to be disposed of.
Filachione, et al. describe a method of preparation of esters from ammonium salts of carboxylic acids; however all the alcohols used had boiling points and overall reaction temperatures well in excess of 100xc2x0 C. (up to 194xc2x0 C.) so as to facilitate the removal of ammonia and water. Filachione, E. M.; Costello, E. J.; Fisher, C. H., xe2x80x9cPreparation of Esters by Reaction of Ammonium Salts with Alcohols,xe2x80x9d J. Amer. Chem. Soc., 5265-67 (1951). Filachione, E. M.; Costello, E. J., xe2x80x9cLactic Esters by Reaction of Ammonium Lactate with Alcohols,xe2x80x9d Ind. Eng. Chem., 2189-91 (1952).
European Patent Appl. EP 517,571 awarded to Alas on Dec. 9, 1992 discloses the synthesis of methyl lactate, butyl lactate, and iso-butyl lactate from optically active ammonium lactate, but does not disclose a synthesis of ethyl lactate.
U.S. Pat. Nos. 5,252,473 and 5,071,754 awarded to Walkup, et al. on Oct. 12, 1993 and on Dec. 10, 1991, respectively, disclose a process for the preparation of ethyl lactate directly from ammonium lactate and ethanol. This method requires the use of CO2 as a catalyst.
U.S. Pat. No. 6,160,173 awarded to Eyal, et al. on Dec. 12, 2000 discloses a process for the recovery of lactic acid esters and amides from aqueous solutions of lactic acids and/or salts.
U.S. Pat. No. 5,723,639 awarded to Datta, et al. on Mar. 3, 1998 discloses a low temperature process for esterifying ammonium and amine-containing salts whereby the salt is reacted with an alcohol in the presence of heat and a catalyst and then subjected to a dehydration and deamination process using pervaporation.
U.S. Pat. No. 5,210,296 awarded to Cockrem, et al. on May 11, 1993 discloses a process for the production of high purity lactate ester or lactic acid from a concentrated fermentation broth by continuous acidification in the presence of alcohol diluent with sequential or simultaneous esterification, distillation off of high purity ester.
U.S. Pat. No. 4,596,889 awarded to Kroener, et al. on Jun. 24, 1986 discloses a process for the preparation of an alkenyl-lactic acid ester by reaction of a cyanohydrin with an alcohol in the presence of hydrogen chloride, followed by hydrolysis.
U.S. Pat. No. 4,314,947 awarded to Hohenschutz, et al. on Feb. 9, 1982 discloses a process for completing the esterification of carboxylic acids with alcohols which employs conventional acid catalysts and water-entraining agents.
None of the aforementioned patents discloses a non-catalytic esterification process.
A need exists in the art for a simple esterification process which does not employ large quantities of solvents, which eliminates competing products, and which is non-catalytic and inexpensive.
It is an object of the present invention to provide a process for the esterification of a carboxylic acid with an alcohol that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a non-catalytic process for producing lactates. A feature of the invention is its operation at or below 100xc2x0 C., often at condenser temperatures of xe2x88x923xc2x0 C. to 15xc2x0 C. An advantage of this invention is that low boiling alcohols (C1-C4) can be utilized under reflux conditions to produce similar carbon length esterified moieties.
Still another object of the present invention is to provide a facile, non-catalytic method for producing relatively pure lactic acid esters from a mixture of carboxylic acid ammonium salts and alcohol. A feature of the invention is the rapid removal of ammonia. An advantage of this method is the facile, non-catalytic synthesis of relatively pure lactic acid esters in high yield.
Yet another object of the present invention is the preparation of carboxylic esters from carboxylic acid ammonium salts and alcohol with high selectivity approaching 100%. A feature of this invention is the elimination of competing species through their rapid removal by the utilization of a chemically inert carrier fluid during refluxing and the use of a drying agent to remove water. An advantage of this process is that it obviates the need for resource intensive removal of such byproducts as amides.
Another object of the present invention is to provide a low-cost process for the preparation of lactates. A feature of this invention is the absence of a catalyst and low operating temperatures. An advantage of this process is that it is simple and inexpensive.
Briefly, the invention provides a non-catalytic process for producing esters, the process comprising reacting an ammonium salt of an acid with an alcohol; removing ammonia from the reaction mixture during refluxing conditions; and removing water from the reaction mixture during refluxing conditions.
The invention also provides a device for esterifying carboxylic acids, the device comprising a condenser; a reaction vessel in fluid communication with said condenser; a soxhlet extractor positioned intermediate said condenser and said reaction vessel, said soxhlet extractor containing a drying agent; a means for injecting inert fluid into the device; and a means for evacuating inert fluid from the device.